Directive antenna system



June 5, 1951 H. IAMS 2,556,087

DIRECTIVE ANTENNA SYSTEM Filed Feb. 2'7, 1948 INVENTOR I garleylamsPatented June 5, 1951 DIRECTIVE ANTENNA SYSTEM Harley Iams, Venice,CaliL, assignor to Radio Corporation of America, a corporation ofDelaware Application February 27, 1948, Ser-ialNo. 11,401

Claims. 1

This invention relates to improvements in ,directive antennas, moreparticularly antennas of the type including a beam-forming reflectorsuch as a paraboloidal sheet. One of the problems which arises in theuse of such antennas is that of feeding or illuminating the reflectorwithout undue distortion of the directive pattern by the masking effector shadow of the feed structure. The difficulty maybe avoided by usingan offset reflector, with the focal point and the feed structure outsidethe main beam. However, this arrangement requires an antenna of greateroverall dimensions than a symmetrical arrangement, for the same beamwidth or gain.

When a reflector which is symmetrical about V the axis of the main beamis usedflthe .feed structure must be located in the path of thereflected energy, and its presence causes a region .of low intensity orshadow in the wave front. The effect is to subtract from theundisturbed. directive pattern .a broad shadow pattern which decreasesthe size of the main pattern lobe and increases the undesired secondarylobes.

It is the principal object of the present .invention to provide methodsand means for compensating or neutralizing the above described shadoweffect.

More specifically, it is an object of this invention to provide improved.feed structures for an tennas of the type including beam-forming re-.flectors.

The foregoing objects are achieved in accordance with the instantinvention by providing the feed structure with means .for radiating aportionof the available powerin the direction opposite the reflector.This power is .made to have an amplitude and phase approximately equalto that masked by the feed device.

The invention will be described with reference to the accompanyingdrawing, wherein:

Figure 1 is a plan view of a directive antenna of the so-called pillboxtype, embodying the invention,

Figure .2 is a sectional view in elevation of the structure of Figure.1,

Figure 3 is .a perspective view of the feedstructure'of the device ofFigure 1,

Figure 4 is a perspective view of .a modification of .theantenna ofFigure l ,,and

Figure .5 .is a view in elevation, partly in section, of .a .rear feedhorn which embodies the present invention.

Referring to Figures 1 and 2, the pillboxincludes two flat parallelmetal sheets I and .3 partially enclosing a relatively narrow strip 5which is curved about a vertical axis to form a short cylindricalparaboloid. A hollow metalwalled wave guide 1 extends vertically downthrough the center of the forward edge of the upper sheet i, terminatingin a right-angle bend 9 at-the center of the rectangular aperture formedby the sheets I and 3 and the stripe. The'mou'th or opening '1 i of theWave guide I is substantially at the focus of the paraboloid formed bythe strip 5.

A narrow horizontal slit i3 is provided in the wall of the guide I atthe bend 9, on the side away from the opening H. The optimum width ofthe slit I3 depends on the relation between the size of the pillboxaperture and the size of the wave guide, but generally will be betweenone-fourth and one-tenth the height of the open end .of the wave guide.A body [5 of dielectric material .such as polystyrene covers the slit.13.

'The thickness of the body 1'5 depends 'upon the described antenna is asfollows:

A radio receiver, not shown, which may comprise merely a detector and anindicator, is coupled to the upper end of the wave guide 1. Atransmitter is set into operation at some relatively remote point, andthe pillbox aligned so as to receive the transmitted signal on thelargest side lobe or secondary lobe of the pillbox directive pattern.The width of the slit is made some arbitrary value, and a tapered wedgeof dielectr-ic material is moved across the'position to be occupied bythe body 15, to determine the thickness which results in a minimumresponse.

The above procedure is repeated, using diiierent slit widths, to obtainthe combination of dielectr'ic thickness and slit width for leastresponse in the largest secondary pattern lobe. To a firstapproximation, this adjustment will minimize all the side lobessimultaneously, while increasing the Sharpness of the main beam.

In the operation of the antenna for transmission, energy is supplied tothe upper end of the waveguide l, and travels to the bend 9, where themost of it emerges from the opening H toward the strip 5. The strip 5reflects and concentrates the energy in substantially parallel rays, asindicated by the .dash lines .in Figure 1. The wave guide! obscures thecentral part of the resulting wavefront, however, tending to widen thebeam. "This effect is overcome by energy radiated from the slit 13. Thedielectric body l delays the energy radiated from the slit l3 by theamount necessary to bring it into phase with that which is masked off bythe wave guide. The overall result is approximately the same as if thewave guide I were transparent to the energy reflected from the strip 5.

It will be apparent that the operation for reception is substantiallythe reciprocal of that for transmission: the received energy which ismasked off by the Wave guide and thus does not reach the reflector strip5 is compensated by energy which enters the slit !3 directly, withoutreflection. Therefore the optimum adjustments of slit width anddielectric thickness for reception are also optimum for transmission.

The antenna of Figure 1 is particularly suited for horizontallypolarized fields, wherein the electric vector is parallel to the sheetsI and 3. The spacing between the sheets I and 3 must be greater thanone-half wavelength, and the propagation through the guide l is in theso-called TEoi mode.

Figure 4 shows an antenna generally similar to that of Figure l butarranged for operation with vertically polarized fields. The pillbox issimilar to that of Figure 1, and its parts are designated bycorresponding reference characters. The feed device in this examplecomprises a coaxial line, with its outer conductor i'i terminating onthe upper sheet i and its inner conductor 19 extending across theopening of. the pillbox at its focus. A cylindrical reflector 2|partially surrounds the exposed lower end of the conductor iii to directradiation therefrom toward the reflecting strip 5.

As in the system of Figure 1, the feed structure obscures the centralpart of the main beam.

'To neutralize the resulting shadow, a vertical slit 23 is provided inthe cylindrical reflector 2 I. This slit is covered by a body 25 ofdielectric material to introduce the required phase delay. The electricfield vector is perpendicular to the sheets i and 3 in Figure 4.Otherwise, the operation and adjustment are like that of the device ofFigure 1.

The invention has been described thus far with reference to pillbox typereflectors comprising cylindrical paraboloids bounded by fiat parallelsheets. It will be apparent, however, that the invention is not limitedthereto, and may be practiced with other types of reflectorconfigurations and other types of feeddevices.

One prior art feed device which presents a partial solution to theproblem of feed masking is the so-called rear feed, wherein a rearwardlydirected radiator is supported on a wave guide extending through theback of the reflector and along the principal directive axis. Astructure of this type, improved in accordance with the presentinvention, is shown in Figure 5. A wave guide 2? extends through theback of a reflector 29, which may be a cylindrical paraboloid, aparaboloid of revolution, or other beam-forming shape, and terminates inan aperture 3! facing away from the reflector. A metal cap 33 issupported on the end of the wave guide 2?, near the aperture 31, and isshaped so as to return or reflect energy radiated therefrom back towardthe main reflector 29. Shadow effect is minimized because the cap 33 andits supporting wave guide 27 may be made small so as to intercept butlittle of the energy reflected from the reflector 29.

The performance of the rear feed structure of Figure 5 is improved, inaccordance with the present invention, by providing a small opening 35in the cap 33, of such dimensions as to radiate about the same power asis intercepted by the feed. As in the other embodiments described above,the phase of the energy radiated through the opening 35 may be delayedby a cover 38 or filling of dielectric material. Alternatively, theopening 35 may be designed to act as a wave guide operating near itscutoff frequency, and introduce an advance in the phase of the energytravelling through it. The length of the passage fiiithrough the cap 33must then be made such as to provide the required change in phase.

Summarizing briefly, the present invention. contemplates the reductionor elimination of shadow effects caused by the presence of the feedstructure within the beam formed by a refiector, by radiation of powerinthe direction away from the reflector, in the same phase and amplitudeas the power masked by the feed.

I claim as my invention:

1. In a directive antenna system including a reflector, a radiatorarranged to illuminate said reflector with energy radiated therefrom andpartially intercepting the energy reflected from said reflector, saidradiator comprising a conductive sheet having a slot therein, said slotbeing positioned to radiate in substantially the same direction as theprincipal direction of reflected radiation from said reflector, andmeans delaying the phase of radiation from said slot to make suchlast-mentioned radiation conform in phase with that intercepted by saidradiator.

2. In a directive antenna system including a reflector, a radiatorarranged to illuminate said reflector with energy radiated therefrom andpartially intercepting the energy reflected from said radiator, saidradiator comprising a conductive sheet having a slot therein, said slotbeing positioned to radiate in substantially the same direction as theprincipal direction of radiation reflected from said reflector, andmeans for making the radiation from said slot conform in phase with thatintercepted by said radiator.

3. The invention set forth in claim 2, wherein the width of said slot issuch that the amplitude of the energy radiated therefrom issubstantially the same as that of said energy intercepted by saidradiator.

4. A directive antenna system including a reilector comprising aparabolic cylinder bounded by fiat parallel plates, a wave guideextending substantially at right angles to said plates and terminatingin an opening at the focus of said cylinder and facing toward saidreflector, said wave guide including a second opening facing away fromsaid reflector, and a body of solid dielectric material through whichenergy from said second opening passes for making the phase of theradiation from said second opening conform in phase with thatintercepted by said wave guide.

5. A directive antenna including two parallel flat sheets of conductivematerial, a strip of conductive material in the form of a paraboliccylinder joining said sheets with its axial plane perpendicular to saidsheets, a wave guide extending substantially along the focal line ofsaid cylinder and terminating in a right-angle bend toward said cylinderand having walls, thatwall portion of said wave guide most remote fromsaid cylinder at said bend including a slot whose width is betweenone-fourth and one-tenth the distance between said fiat sheets, and asheet of dielectric material covering said slot.

6. A feed structure for a paraboloidal reflector, comprising a hollowwave guide terminating in an opening which faces away from thereflector, a conductive body adjacent said opening and formed to directradiation therefrom toward said reflector, said body including anopening which faces away from said reflector, and means altering thephase of energy radiated through said last-mentioned opening.

7. A feed structure for a paraboloidal reflector comprising a radiator,means directing the maior portion of radiation therefrom in onedirection for reflection by said reflector, said means causing ashadowed spaced in the radiation patternof said reflector with saidmeans, and means directing a relatively small portion of the radiationfrom said radiator in the opposite direction into the shadowed space,said last-mentioned means including means introducing a predetermineddelay in the phase of said radiation therefrom.

8. A feed structure for directive antenna systems having a reflector,comprising a hollow rectangular wave guide having an opening facing saidreflector, a flat sheet of conductive material at one end of said waveguide and at an angle or 45 degrees to the direction of propagation ofenergy therein and in a position at the end of said wave guide to directenergy therefrom to said reflector, the radiation pattern of saidreflector "and feed structure having a shadowed space, said sheetincluding a slot lying perpendicular to said direction of propagationand having a width between one-fourth and one-tenth the width of saidwave guide radiation through said slot from the side of said sheetfacing said reflector to the other side illuminating said shadowed spaceto compensate for the shadow, and a body of solid dielectric materialpositioned to be traversed by energy passing through said slot and notby energy not passing through said slot.

9. A directive antenna system comprising, a reflector, a feed structurearranged to illuminate the reflector as a transmitting antenna orreceive energy reflected therefrom as a receiving antenna, said feedstructure including a transmission line having a portion forming ashadowed space in the radiation pattern of the reflector, saidtransmission line having a radiator directing energy away from saidreflector into said shadowed space as a transmitting antenna orintercepting energy directed toward said reflector as a receivingantenna from said shadowed space, thereby to compensate for the shadowof the radiation pattern without said radiator.

10. The antenna system claimed in claim 9, said transmission lineradiator comprising a slot in a conductor of said transmission line.

HARLEY IAMS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,129,712 Southworth Sept. 13,1938 2,273,447 Ohl Feb. 17, 1942 2,407,318 Mieher et a1. Sept. 10, 19462,422,184 Cutler June 17, 1947 2,429,640 Mieher et al Oct. 28, 19472,436,408 Tawney Feb. 24, 1948 2,447,549 Willoughby Aug. 24, 1948

